Heat the milk in a small saucepan until it bubbles, then remove from heat. Add the butter, stir until melted and pour into a large bowl to cool. Dissolve yeast in the milk mixture. Mix in 1/3 cup sugar, salt, eggs and 2 cups flour. Stir in the remaining flour, 1/2 cup at a time, beating well after each addition. When the dough has pulled together, turn it out onto a lightly floured surface and knead until smooth and supple, about 8 minutes. Lightly oil a large bowl, place the dough in the bowl and turn to coat with oil. Cover with a damp cloth and let rise in a warm place until doubled in volume, about 1 hour. Divide the dough into 16 equal pieces and form into rounds. With a rolling pin, roll the rounds into oval buns. Place them onto two lightly greased baking sheets. Cover the rolls with a damp cloth and let rise until doubled in volume, about 40 minutes. Meanwhile, preheat oven to 350 degrees F (175 degrees C). While the bread rises, make the topping: In a small bowl, stir together 1/2 cup sugar, 2/3 cup flour, butter until a crumb mixture forms. Stir in the egg yolks until well blended. Crumble the topping over the rolls so the entire surface is covered. Bake in preheated oven for 15 to 20 minutes, or until sugar topping is lightly browned.